1. Field of the Invention
This invention relates to carriers for polishing double sides of work pieces, held in work retainer holes, as it is rotated and revolved relative to two polishing pads between which it is interposed, and more particularly, to carriers for polishing double sides of semiconductor wafers.
2. Description of the Prior Art
Work pieces having been flattened through lapping process are subjected to a wet etching process to remove a residual damaged layer resulting from the lapping process, and then subjected to a polishing process, which permits highly accurate flatness in addition to mirror finish. The polishing process is carried out by a mechano-chemical polishing method comprising a plurality of stages.
The mechanical polishing provides a "scraping-off effect" and a "surface atom arrangement disturbance effect". The chemical polishing, on the other hand, provides a "solving effect" and a "film formation effect" to the surface of work pieces. These effects constitute a composite effect of permitting highly accurate mirror finish. At any rate, these effects are influenced by what extent is put emphasis on whether a mechanical factor or chemical factor during polishing.
In the meantime, polishing process includes the step to polish coarse surface to mirrored surface and the succeeding step to continue polishing the said mirrored surface in order to approach the necessary flatness. Particularly, semiconductor materials such as high performance products require minimized damaged layer as well as the mirror-finished surface. In other words, it is required to obtain a predetermined accurate surface flatness. The processed surface and the layer directly under the processed surface require the exactly same state as the inner part of the wafer.
For such process, double-side polishing machines for polishing double sides of disc-like work pieces are used to improve efficiency in the process.
FIGS. 9(A) and 9(B) show a conventional double-side polishing machine. As shown in FIG. 9(A), the polishing machine comprises a disc-like carrier 10, which has work retainer holes 11 formed in it and a peripheral gear 10a formed around its outer periphery. As shown in FIG. 9(b), the peripheral gear 10a is meshed with a sun gear 53, which is formed on the center of a lower polishing turn table 51 rotated in the direction of arrow A, and also with an internal gear 54 provided on the outer side of the lower polishing turn table 51.
The carrier used in the above conventional double-side polishing machine is the same in outward shape, number of work retainer holes as the carrier according to the present invention. For this reason, the same reference numerals and symbols are used as for the carrier 10, work retainer hole 11 and peripheral gear 10a in the above description, are used in the description of the present invention.
The carrier 10 actually has three work retainer holes 11. Wafers 25 as work pieces are inserted and held in the work retainer holes 11. In this state, the wafers 25 are held clamped under a proper pressure between polishing pads 51a and 52a, which serve as polisher and are applied to lower and upper polishing turn tables 51 and 52 rotated in opposite directions, so as to polish double sides of the wafers at a time by dropping predetermined abrasive slurry through an abrasive slurry feed hole 56 formed in the upper polishing turn table 52.
The carrier may be a metal body. As an example, Japanese Laid-Open Utility Model Registration No. 58-4349 proposes a resin-coated metal carrier. Carriers of other materials also have been proposed. For example, Japanese Laid-Open Patent Publication No. 58-143954 proposes a carrier which is a resin-impregnated carbon fiber laminate.
In the meantime, the high quality mirror finish mentioned above requires the use of very fine abrasive grains and a soft polisher (i.e., polishing pads). Another important factor to obtain good finish is the wear of the polishing pads. It was observed that polishing pads are worn out harshly at an initial stage of polishing, during which the wafers have considerably rough surfaces, but the wear of the polishing pads is suppressed with the progress of flattening the wafers. It is possible to take the view that in the removal of material by polishing not only the behavior of abrasive grains is concerned, but also a mechanical effect of "scraping-off" provided by the polishing pads is inevitable.
Furthermore, it is considered that a polishing mechanism is provided that a soft film (or hydrated film) formed by a chemical action, is scraped off and removed by abrasive grain and/or polishing pads as polisher. The above polishing mechanism is provided when mechanically polishing silicon wafers with colloidal silica. In this case, a combined effect of fine abrasive grains and soft polisher is provided, so that the silicon surface is not directly rubbed off, but the processing proceeds with the removal of the soft film (or hydrated film). It is thus possible to obtain non-disturbed mirror finish free from processing defects.
At any rate, wear of the polishing pads as polisher is inevitable. When a worn-out polisher is used, it is elastically deformed by the work pushed against it, thus resulting in a polished surface having a convex shape.
The amount of polishing is increased with the lapse of polishing time, and the flatness is deteriorated with increasing polishing amount. Therefore, it is necessary to correct the flatness of the polishing pads as polisher.
The abrasive grains used are very fine, i.e. 1 .mu.m or below, while the polisher is formed by using soft materials, such as synthetic resins or fibers. During polishing, polishing reaction products, removed by polishing from the work surface by abrasive grains, are dispersed in the polishing slurry and partly stick to the surface of the polishing pads, thus deteriorating the polishing performance. To remove the stuck matter, the polishing pads should be dressed.
The polishing pads are dressed by, for instance, brushing of them with brush, which is done at an adequate frequency, or their dressing done by inserting dressing grindstones.
However, the frequency of dressing the polishing pads, even set adequately, is greatly varied according to the extent of sticking of reaction products to the polishing pad surface, which is in turn dependent on characteristics fluctuations of the polishing pads caused by in the manufacturing process thereof.
Therefore, it is necessary to determine the frequency of carrying out the dressing of the polishing pads by confirming the polishing accuracy of the polished work, and this gives rise to problems in view of the production efficiency.
Furthermore, depending on the kind of the polishing pads it is necessary to make dressing whenever the polishing is ended. The operation of dressing the polishing pads is made by removing the carrier carrying the work and inserting a carrier holding grindstone between the polishing pads, therefore it greatly reduces the production efficiency.